1) Field of the Invention
The present invention relates to an apparatus for controlling motor toque that controls torque of an electric motor of an electric vehicle or a hybrid electric vehicle when the motor generates creep torque.
2) Description of the Related Art
Recently, electric vehicles and hybrid electric vehicles that use electric motors (motors) as traveling sources have been developed and put into practical use in place of gasoline automobiles, which use internal combustion engines as traveling sources. An electric vehicle can arbitrarily control torque of the electric motor and can therefore controls, when the vehicle stops, the motor not to generate motor torque so that electric consumption can be saved. Also a hybrid electric vehicle, when the vehicle stopping, generates no motor torque to save electric consumption and concurrently stops the engine under so-called idle stop control.
In the meantime, a gasoline automobile including automatic transmission equipped with a conventional torque convertor generates creep torque caused from idling rotation of the engine when the engine rotates at a low speed. The creep torque allows the vehicle to travel at a low speed when parking and traveling in a traffic jam, only brake operation, i.e., not requiring accelerator operation. In addition, the creep torque can prevent the vehicle from moving backward when the brake is released for starting the vehicle on an upward slope, so that the operatively of the vehicle can be enhanced. At the present time, most vehicles put into the market are capable of creeping and therefore creeping is an necessary function for driving.
For the above, there has been developed a technique to generate creep torque by driving a motor of an electric vehicle and a hybrid electric vehicle. In this technique, generating motor torque when the brake pedal is pressed leads to wasting electric power. In order to avoid wasting electric power, the most effective solution is to make the motor torque zero under a state where the brake pedal is pressed. On the other hand, in response to a demand for creep torque, it is required to generate motor torque to satisfy the demand.
In relation to the above, Patent Literature 1 (Japanese Laid-Open Patent Publication No. 2004-320850) proposes a technique to generate motor torque corresponding to a status of pressing the brake. In this technique, when the vehicle speed is a predetermined value or less, when the accelerator opening is substantially zero, and when a status of pressing the brake is in the direction of increasing the braking force, the motor torque is rapidly reduced to zero while, when the status of pressing the brake is in the direction of decreasing the braking force, motor torque corresponding to an amount of depressing the brake is generated. Thereby, it is possible to save a waste electric consumption. Besides, since, when the vehicle starts moving on a steep upward slope, easing up on the brake causes the status of pressing the brake to be in the direction of decreasing the braking force, it is possible to prevent the vehicle from moving backward by generating motor torque corresponding to the amount of pressing the brake.